1. Field of the Invention
The present invention generally relates to an alternating-current (AC) high-voltage device that outputs an AC high-voltage, and specifically relates to an AC high-voltage device that outputs a constant AC high-voltage.
2. Description of the Related Art
As disclosed in Japanese Patent No. 3167092 and Japanese Patent Application Laid-open No. H8-101589, an electro-photographic image forming apparatus such as a printer, copier, facsimile, or Multifunction Product (MFP), forms a toner image on an image carrier and transfers the toner image onto such an image medium as a transfer sheet applying a charge bias scheme. When transferring though this scheme, the image media are attracted to each other after transfer by electrostatic-force. To avoid this problem, a scheme is used for separating these image carriers by eliminating static charge through AC high-voltage discharge.
Particularly in so-called tandem color image formation, an attachment force is strong between the transfer sheet and an intermediate transfer belt after an image-transfer, where images generated on photosensitive members for respective colors are combined to form a color image in the course of transfer onto an intermediate transfer belt and then the image is transferred from the intermediate transfer belt to a transfer sheet. Therefore, it is indispensable to ensure sheet separation by applying an AC high-voltage output to a static charge eliminator for discharge onto a paper sheet subjected to paper transfer, which is shown in Japanese Patent Application Laid-open No. H8-101589.
In this case, an AC high-voltage output applied to the static charge eliminator has a large-power peak-to-peak (hereinafter, “Pk-Pk”) voltage of approximately 8.0 kilovolts to 12.0 kilovolts. Therefore, to ensure safety, a unit is provided that detects an over-current in AC high-voltage output to an AC high-voltage generating unit, making it possible to avoid a failure due to over-current.
It is assumed herein that an over-current is detected in AC high-voltage output on condition that a value approximately double a specification range of an output current in AC high-voltage output is detected successively for approximately 200 milliseconds to 300 milliseconds. If the detection condition is satisfied, the AC high-voltage output is forcefully turned OFF by an AC high-voltage unit. Information indicative of an abnormality is fed back to a main controlling unit of the image forming apparatus, causing a signal of a return-to-normal-state be issued as a serviceperson call (SC) from the image forming apparatus.
FIG. 9 is a signal line diagram for explaining a related art of an AC high-voltage generating unit. In this drawing, the vertical axis represents an output value, and the horizontal axis represents time, and depicted is a relation among three signals on the same time axis, that is, (A) a pulse-width modulation (PWM) signal, (B) an output current of an AC high-voltage output, and (C) an SC signal.
The AC high-voltage generating unit, which generates an AC high-voltage output to supply the static charge eliminator, controls voltage to be constant for a transformer by a target Pk-Pk value indicated by the PWM signal from a main controlling unit of the image forming apparatus, thereby transforming supplied power to an AC high-voltage output. An over-current in an output current of the AC high-voltage output which is detected as a value approximately double the specification range of an output current detected successively for approximately 200 milliseconds to 300 milliseconds. Therefore, a current exceeding an over-current detection value flows, as shown in FIG. 9, if only continued for 200 milliseconds to 300 milliseconds output, and then the AC high-voltage output is turned OFF.
When turning the output OFF, an SC signal indicative of an abnormality is sent from the AC high-voltage generating unit to the main controlling unit.
The main controlling unit detects the SC signal that addresses the abnormality, and turns OFF the PWM signal sent to the AC high-voltage generating unit.
In the configuration, even after turning the PWM signal OFF, the SC signal continues to output indicating an abnormality output from the AC high-voltage generating unit, while the output current is being turned OFF without being cancelled to recover until the power of the image forming apparatus is turned OFF/ON.
However, the technology explained with reference to FIG. 9 has following problems (1) to (3):
(1) The AC high-voltage output to be applied to the static charge eliminator is sensitive to a conductive substance attached to paper and to a momentary disturbance noise, leading to erroneously detect a previously-set over-current for the AC high-voltage output. Therefore, even a normal safe situation, an SC erroneous detection tends to occur.
(2) To reduce such an SC erroneous detection, if the over-current detection condition of the AC high-voltage output is simply eased by, for example, increasing the over-current detection value or the detection time of the AC high-voltage output, there is a possibility, for example, that smoking or ignition may occur when a user inadvertently sets a conductive paper sheet, such as a light-shield paper sheet for shielding a photosensitive member. In this case, safe operation cannot be guaranteed.
(3) When the AC high-voltage output meets the over-current detection condition, the AC high-voltage generating unit forcefully turns the AC high-voltage output OFF, and feeds an SC signal indicative of an abnormality back to the main controlling unit. The image forming apparatus then detects the SC. Then, the SC signal is continued to be in that state until the image forming apparatus is powered OFF/ON, and cannot be cancelled, which means a recovery cannot be operated. Therefore, even if a system reset of the image forming apparatus works, the SC signal cannot be cancelled. Thus, it disadvantageously takes time to recover.